As is known in the art, a polarographic sensor comprises an anode and cathode which are electrochemically coupled, via an electrolyte, which is pH-sensitive to the determined gas. Specifically, the pH of the electrolyte is generally proportional to the concentration of the determined gas contained in the gas sample to which the sensor is exposed. Because the current flow from cathode to anode varies with the pH of the electrolyte, one may theoretically impress a constant voltage between the two electrodes and monitor the changing current to determine the pH and, accordingly, partial gas pressure.
The present invention is more particularly related to a method and apparatus for automatically calibrating medical devices employing a polarographic sensor. The sensor's properties change over time, adversely effecting the correlation between pH and gas pressure. These changes are caused, for example, by ambient conditions such as barometric pressure, temperature, and relative humidity. Long-term changes also occur, owing to, inter alia, the drying out of the electrolyte.
Because of the complex chemical-physical relationship underlying the relationship between electrolyte pH and partial pressure of the gas to be determined, the calibration procedure and calibration maintenance of such instruments is a time-consuming operation requiring skilled personnel.